Heretofore, electronic devices have been available for measuring surface temperature, i.e. thermocouples, infrared detectors, thermistors. These devices are costly to purchase and maintain, required batteries or an external power source, were bulky, and easily damaged.
Thermochromic devices such as shown in U.S. Pat. No. 3,002,385 to Wahl, U.S. Pat. No. 2,928,791 to Laconti, and U.S. Pat. No. 4,137,769 to Parker, avoid many of these problems. However, these devices are permanently bonded to the surface in order to provide the required for good thermal contact. The devices may not be removed, and remounted on another site.
U.S. Pat. No. 3,955,420 and U.S. Pat. No. 3,827,301, both to Parker teach a rod thermochromic device with cooling fin heat exchange characteristics. The tip of the device is pressed against the surface to be measured. A temperature gradient is established along the device causing a progressive thermochromic change extending outward from the contact end. The surface temperature is indicated by the extend of the thermochromic change present at equilibrium. This rod device requires many seconds to reach the equilibrium required for accurate readings. The time element is especially burdensome when measuring high surface temperatures which have a correspondingly high transient time.